Synthetic resins are widely used in various industrial fields. Among the synthetic resins, an industrially manufactured polyester, especially polyethyleneterephtalate (hereinafter referred to as PET) has superior physical and chemical features and is used as fibers, films, and other molded articles. For example, in a film area, the polyester is used for magnetic tapes such as audiotapes and videotapes, condensers, photographs, wrappings, OHP, pre-paid cards and so on.
Slipperiness and anti-shavingness of the polyester film are main features that control workability in a manufacturing step or a processing step of the film in various uses, and a quality of a product. In a case where a magnet layer is coated on a surface of the polyester film so as to use as a magnetic tape, if the slipperiness and anti-shavingness of the film are not sufficient, a friction between coating rolls and the surface of the film becomes large, thereby causing a severe abrasion of the surface of the film, and in an extreme case, resulting in wrinkles and scratches on the surface of the film. Even after processing the film into a tape such as audiotapes, videotapes and computer-tapes by slitting the film which is coated with the magnetic layer, an extreme friction between the surface of the film and many guides or a reproduction head will occur during operations such as drawing out of the tape from a reel or cassette, winding the tape and so on. This extreme friction causes scratches or strain, and the shaving of the surface of the polyester film deposits white powder, and results in a lack of magnetic recording signal, namely, drop-out.
For lowering a friction coefficient of the polyester, many methods for improving slipperiness of the surface of the molded articles are proposed by incorporating inorganic fine particles into polyester so as to give minute and appropriate roughness to the surface of the molded articles of the polyester. However, since affinity between the fine particles and the polyester is not sufficient, both transparency and anti-abrasion ability of the film are not satisfactory.
In case of a polyolefin, the polyolefin is widely used in various industrial fields. Especially, a polyolefin film such as a polypropylene film is the most widely used as various wrapping materials. Since it is known that such kind of the polyolefin film is adhesive, the polyolefin film tends to cause blocking. For this reason, workability in a process for manufacturing or processing of the film becomes worse. Furthermore, in a case where the film is used in packaging or wrapping, troubles such as less workability in opening the package or wrapping tend to occur. Therefore, in general, an anti-blocking treatment is done on this kind of film. As an anti-blocking agent, a finely powdered silicic acid, zeolite, calcium carbonate and kaolin cray are typically known and used.
On the other hand, the polyolefin film requires, as qualitative features, superior transparency and good anti-scratch ability (For example, the surface of the film is less scratched when the surface of the film is contacted with each other.). However, the transparency and the anti-scratch ability, are contrary to the anti-blocking ability. For example, in order to improve the anti-blocking ability of the polyolefin film, if a large amount of the blocking agent is used, the anti-scratch ability and the transparency become poorer as an amount of the blocking agent to be added is increased. Therefore, as the additive to improve the anti-blocking ability, the anti-scratch ability and the transparency of the film to a satisfactory level, the conventional inorganic powder was far from satisfaction.
In case of the conventional kaolin cray, since the particle shape of the kaolin cray has a plate-like structure, when the kaolin cray is used as a blocking agent of the polyolefin film, the kaolin cray can not form roughness on the surface of the polyolefin film in a satisfactory level. Therefore, a large amount of kaolin cray is required to obtain a satisfactory level of anti-blocking ability. As a result, only polyolefin film having insufficient transparency could have been obtained.
In case of the finely powdered silicic acid, since the main particles of the silicic acid is extremely minute, a polyolefin film with a satisfactory level of transparency and anti-scratch ability can be obtained. However, since the silicic acid cannot form roughness on the surface of the polyolefin film, a polyolefin film having a satisfactory level of anti-blocking ability could not have been obtained even though a large amount of silicic acid is used.
In case of the powdered zeolite, a polyolefin film that has good transparency and anti-blocking ability can be obtained when compared to a film containing the kaolin cray or the finely powdered silicic acid. However, a film with good anti-scratch ability cannnot be obtained. Moreover, since the zeolite is known to have crystalline water, the zeolite may cause defective products due to bubbling formed by the crystalline water which is released from the zeolite by heating in a process of molding a synthetic resin or making a film. If the zeolite is heated to remove the crystalline water (so called zeolite water) so as to make an activated zeolite which does not have crystalline water, the zeolite readsorbs water easily. Therefore, it is substantially impossible to remove an influence of the water in a process for film formation.
Furthermore, in case of the calcium carbonate, since the calcium carbonate does not have crystalline water, there is no bubbling due to a release of the crystalline water. However, since the calcium carbonate inherently has a strong aggregation tendency, the calcium carbonate tends to form secondary large particles which are formed by aggregation of many primary particles. Therefore, the calcium carbonate has problems to be solved as a good blocking agent for a polyolefin film with a good anti-blocking activity, transparency and anti-scratch ability.
In view of the above facts, after extensive and intensive studies on an additive for synthetic resins which can give a good anti-blocking ability, good transparency and good anti-scratch ability to synthetic resins represented by polyesters or polyolefins, especially a film or a fiber thereof and which has a good affinity to the synthetic resin, and further on a synthetic resin composition containing the additive, the present inventors have found out that particles having a specific particle composition, specific particle diameter and distribution thereof and specific surface have functions as the intended additive for synthetic resins, and that a synthetic resin composition containing this additive can attain the intended purpose, and thus accomplished the present invention.